The kinetic energy of a rolling billiard ball is given by {\rm{KE}} = 1/2\,mv^2 . Suppose a 0.17-{\rm kg} billiard ball is rolling down a pool table with an initial speed of 4.5 m/s. As it travels, it loses some of its energy as heat. The ball slows down to 3.5 m/s and then collides straight-on with a second billiard ball of equal mass. The first billiard ball completely stops and the second one rolls away with a velocity of 3.5 m/s. Assume the first billiard ball is the system